Method for synchronizing the internal clock of a mobile radio terminal with local time

ABSTRACT

The invention relates to a method for synchronizing the internal clock of a mobile radio terminal with the local time, which is characterized in that the mobile radio terminal transmits a short message (SM) to itself, the local time may be deduced from the transmission and received times for the short message signal and the time stamp within the received short message and the internal clock is synchronized with the said local time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the synchronization of the internal clock of a mobile radio terminal device with the local time.

2. Description of the Prior Art

It is known that a mobile radio terminal device, in particular a GSM terminal device, does not have the capability of determining the correct local time of day. Even if a clock module is installed in the terminal device, it is, in general, necessary for the user to provide it with the correct time.

The invention addresses the problem of proposing a simple and automated method for the synchronization of the internal clock of a mobile radio terminal device with the local time.

SUMMARY OF THE INVENTION

This problem is solved in the invention as herein disclosed and claimed.

The described method offers the mobile radio subscriber the capability of having displayed the current local time of day by the mobile radio terminal device in simple manner. The time of day can be determined accurately to a few seconds, which, for practical purposes, is entirely sufficient.

The method can be expanded such that a mobile radio subscriber can be provided with the “correct” local time even when traveling, i.e. when roaming, virtually by the “push of a button”.

BRIEF DESCRIPTION OF THE DRAWINGS

In conjunction with FIGS. 1 and 2 an example of the method according to the invention will be explained. Based on the Figures and their description, additional characteristics and advantages of the invention will become evident. The drawings depict:

FIG. 1 by way of example a flow chart of the method for setting the clock in the terminal device by means of a short message to itself,

FIG. 2 flow chart for improving the accuracy of the method.

As is evident in FIG. 1, the method according to the invention essentially utilizes the fact that in digital mobile radio networks, in particular GSM mobile radio networks, each short message is conducted via a so-called Short Message Service Center (SMSC), and here each message is provided with a time stamp t_(Z) which, in turn, is indicated to the receiver of the message.

Thus, if a mobile radio terminal device sends a short message to itself at time t₀, it is informed at time t₂ (after a few seconds) of the time t_(Z) of the arrival of its own message in the SMSC when receiving the short message sent to itself. If the terminal device remembers the sending time t₀ by registering the setting of the internal clock, as well as the acknowledge time t₁, a few seconds after sending its own message, then thereby a time window between t₀ and t₁ is given, in which the time stamp t_(Z) of the SMSC must assuredly lie. Through maximal boundary definition of the time window, the time t_(Z) can be derived with good accuracy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed consideration of the minimum transmission times in the GSM network, in particular the delay time of the signals at the air interface, permits a further shrinking of the time window between t₀ and t₁, such that an accuracy of the prediction of t_(Z) of approximately one second appears attainable. This accuracy is often sufficient for practical purposes. This will be explained in the following in conjunction with FIG. 2.

At time t₀ the terminal device sends a short message to itself, which is received by the SMSC.

The terminal device calculates the time t₀′=t₀+d₀ wherein d₀ is the estimated or known minimum transmission time for the message from the terminal device to the SMSC. The message cannot have arrived at the SMSC before the passage of d₀ time units.

The terminal device waits for the acknowledgment (Ack) of the reception of the message in the SMSC. The acknowledgment arrives at time t₁.

The terminal device calculates the time t₁′=t₁−d₁, wherein d₁ is the estimated or known minimum transmission time for the acknowledgment from the SMSC to the terminal device. The SMSC must have gotten the acknowledge on its way before this time t₁′.

Now, the mean value t_(M)=(t₁′+t₀′)/2 is formed from the calculated times t₀′ and t₁′. This mean value t_(M) corresponds fairly accurately to the value (point in time) of the time stamp t_(Z) assigned by the SMSC.

The terminal device now waits for the arrival of the message directed to itself, which arrives at time t₂.

The terminal device takes the time stamp t_(Z) from the message.

The terminal device compares the time stamp t_(Z) with the calculated value t_(M) and, if necessary, corrects its internal clock by the difference of t_(D)=t_(Z)−t_(M) time units. It is obvious that with this selection of correction the maximum time error relative to the SMSC is t=(t₁−t₀)/2 or the least.

The absolute accuracy depends on the maximum discrepancy of the SMSC clock from the “true time”, i.e. the absolute time error is greater by the inaccuracy of the time base in the SMSC.

The method can also be applied in the case of roaming, i.e. if a mobile radio subscriber is located outside of his home network, if the SMS message is transmitted to a time server connected to the SMSC, and the time server is informed in the message of the network code of that network into which the terminal device is currently logged in. The time server provides in his response to the terminal device information about the time zone or the correction to be applied. Herein the home SMSC can continue to be employed. This can be realized relatively readily in the terminal device, for example through an SIM toolkit application. 

1. Method for the synchronization of the internal clock of a mobile radio terminal device with the local time, wherein the mobile radio terminal device sends a short message (SM) to itself and from the sending and receiving times of the short message signals and a time stamp t_(z) comprised in the received short message derives the local time of day and synchronizes the internal clock with this local time of day, and wherein the mobile radio terminal device registers the sending time t₀ of the short message as well as the time of acknowledge t₁ of the short message and derives therefrom the time t_(M)=(t₁+t₀)/2, wherein t_(M) corresponds approximately to the point in time of the time stamp t_(z).
 2. Method as claimed in claim 1, wherein the mobile radio terminal device extracts from the received short message the time stamp t_(z), compares it with t_(M), and corrects the internal clock by the difference t_(D)=(t_(z)−t_(M)).
 3. Method as claimed in claim 1, wherein an improvement of the accuracy of t_(M) is attained by taking into consideration the delay times d₀ and d₁, with t_(M) =(t₁′+t₀′)/2, wherein t₁′=t₁−d₁ and t₀′=t₀−d₀.
 4. Method as claimed in claim 1, wherein the SMSC is informed in the short message of the network code of that network into which the terminal device is currently logged in, which network code is transferred to a time server connected to the SMSC and the time server transmits to the terminal device in its response the time zone or the correction to be applied. 